1. Field of the Invention
The present invention is related to a transmission system comprising a transmitter for transmitting a sequence of digital symbols to a receiver, said receiver comprising means for deriving at least a first detection signal and a second detection signal from the received signal, at least a first detector for deriving a first sequence of detected symbols from the first detection signal and a second detector for deriving a second sequence of detected symbols from the second detection signal, and means for determining an output sequence of digital symbols from at least the first and second sequence of detected symbols.
The present invention is also related to a receiver and to a receiving method.
2. Discussion of the Related Art
A transmission system according to the preamble is known from the article "performance improvement in decision feedback equalisers by using soft decision" by E. Dahlman and B. Gudmundson in Electronics Letters, Vol. 24, No. 17, Aug. 18, 1998.
Transmission systems of this type may be used, for example, for digital symbol transfer through the public telephone network or for reconstructing digital symbols received from a magnetic tape or disc.
When digital symbols are transmitted from a transmitter to a receiver or stored on a recording medium respectively, the symbols to be transmitted or recorded are converted to a signal representing the digital symbols. This generally happens in the form of analog pulses which are successively applied to the transmission medium or recording medium respectively, further to be referenced by the term channel. At the input of the receiver, analog pulses are present. The receiver comprises means for deriving a detection signal from the input signal. This means can comprise an equalizing filter, it can comprise a subtracter for subtracting postcursive intersymbol interference derived from previously detected symbols, or it can simply comprise an interconnection. The detector is arranged for deriving a sequence of detected symbols from the detection signal. The detector may comprise a simple comparator for comparing the detection signal with one or more reference levels.
In addition to the (desired) analog pulses, there is nearly always an undesired noise signal present in the output of the channel. Due to the presence of this noise signal, the detector will occasionally make erroneous decisions with respect to the value of the transmitted symbols. In order to reduce the adverse influence of said erroneous decisions the receiver in the transmission system according to the above mentioned article comprises so-called erasure detection means for determining whether the analog pulse has an amplitude in the vicinity of a reference level. If this is the case, a decision based on this pulse has a reduced reliability. To deal with this reduced reliability, the above mentioned receiver comprises two detectors. In the transmission system according to the above mentioned article, binary signaling is used. In the case of an unreliable decision, the decision of one detector, is reversed. The result is that, at the output of one of the detectors a correct sequence of digital symbols is present. In the means for determining an output sequence of digital symbols it is checked which sequence has the largest likelihood to be correct. This sequence is subsequently selected as the output sequence.
In the above mentioned transmission system, the maximum attainable data rate is limited, due to the time required for detection of an erasure and the introduction of an additional switching delay needed to invert the output signal of the second detector in case of an erasure.